1. Field of the Invention
This invention relates to an edge achromatization circuit and method for digital camera signal processing, and more particularly to an edge achromatization circuit and method by which a false color at an edge of an image in digital camera signal processing in which a single plate image pickup element is used is removed.
2. Description of the Related Art
Conventionally, in a digital camera which uses an image pickup element of a single plate, a false color which appears at an edge of an image becomes a problem which cannot be ignored as the picture quality of a camera becomes higher. Thus, it is requested to suppress a false color to obtain a high picture quality.
In order to satisfy the request, it is proposed to suppress the amplitude of a chroma signal at an edge portion of a luminance signal to suppress a false color at an edge of an image as disclosed, for example, in Japanese Utility Model Laid-Open No. 070773/1988 entitled “Achromatization Edge Processing Circuit”.
FIG. 6 shows a general configuration of the achromatization edge processing circuit disclosed in the document specified above. Referring to FIG. 6, the achromatization edge processing circuit shown includes an image sensor (CCD) 45, amplifiers (AMP) 46, 52 and 53, an automatic gain control and γ conversion circuit (AGC/γ) 47, low-pass filters (LPF) 48 and 49, a band-pass filter (BPF) 50, an edge extraction circuit 51, a color separation circuit 54, a white balance (WB) circuit 55, a chroma modulation circuit 56, a YC mixture circuit 57, and an output terminal 58.
An output of the image sensor 45 is subject to gain control and γ correction by the amplifier 46 and the automatic gain control and γ conversion circuit 47. An output of the automatic gain control and γ conversion circuit 47 is band-limited by the low-pass filter 48 so that it is converted into a luminance signal YH which includes many high frequency components, and is band-limited by the low-pass filter 49 so that it is converted into a luminance signal YL which does not include many high frequency components. Further, the output of the automatic gain control and γ conversion circuit 47 is band-limited by the band-pass filter 50. An output of the band-pass filter 50 is converted into color signals R and B by the color separation circuit 54.
The color signals R and B are subject to color balancing and arithmetic operation with the luminance signal YL, which does not include many high frequency components, by the white balance circuit 55 so that they are converted into color difference signals R-Y and B-Y, respectively. The color difference signals R-Y and B-Y are converted into a chroma signal of a video signal by the chroma modulation circuit 56 and then inputted to the amplifier 53. Meanwhile, the luminance signal YH which does not include many high frequency components is converted into an edge signal by the edge extraction circuit 51, and the edge signal is shaped by the amplifier 52 and then inputted to the amplifier 53. The amplifier 53 performs gain control of the chroma signal so that, as the magnitude of the edge signal decreases, the amplitude of the chroma signal may increase, and as the magnitude of the edge signal increases, the amplitude of the chroma signal may decrease.
In this manner, the achromatization edge processing circuit disclosed in the above-specified document extracts an edge component from a luminance component which includes a comparatively small amount of high frequency components and performs gain control of a chroma signal with the extracted signal to suppress a false color.
In the achromatization edge processing circuit disclosed in the document mention above, however, since gain control for a chroma signal is performed with an edge signal of a luminance, processing is performed for a signal having a non-linear characteristic to which γ correction has been performed. Therefore, the achromatization edge processing circuit has a problem that, when the signal is regenerated finally as an image, the image is suppressed unnaturally.
Further, where a color difference signal is used for interfacing, a chroma signal must be decoded back into color difference signals. If the chroma signal is that of the NTSC system, then since the color difference signals are subject to band-limitation by the low-pass filter and the band-pass filter, the achromatization edge processing circuit has another problem that a signal band for a color disappears and the resolution is deteriorated.
Further, since the characteristic that the gain is lowered when the level of the edge signal is high is reverse to that of multiplication, the achromatization edge processing circuit has a further problem that, where digital signal processing is applied, a large circuit scale is required.